Generally, electrolytic capacitors comprise an anode body of a valve metal such as Al (aluminum) or Ta (tantalum), a dielectric oxide film formed over the anode body by an electrolytic oxidation treatment, and a cathode layer of electrolyte, MnO.sub.2 (manganese dioxide), conductive organic compound or like conductive substance formed over the oxide film in intimate contact therewith. The term "valve metal" as used herein refers to a metal which forms a highly compacted durable dielectric oxide film when subjected to an electrolytic oxidation treatment. Such metals include Ti (titanium) and Nb (niobium) in addition to Al and Ta. Since the dielectric oxide film has a very small thickness, it is possible to prepare electrolytic capacitors having a smaller size and greater capacity than other capacitors such as paper capacitors and film capacitors.
Electrolytic capacitors wherein a solid conductive substance, such as MnO.sub.2 or conductive organic compound, is used for the cathode layer are called solid electrolyte capacitors. Among these, those wherein a solid conductive organic compound is used are termed organic solid electrolyte capacitors. Examples of such conductive organic compounds are polypyrrole, polyaniline and like conductive high-molecular-weight compounds and TCNQ (7,7,8,8,-tetracyanoquinodimethane) complex salts.
These conductive organic compounds are higher than electrolytes and MnO.sub.2 in electric conductivity. Accordingly, the organic solid electrolyte capacitors wherein the conductive organic compound is used for the cathode layer are lower in ESR (equivalent series resistance) and more excellent in high-frequency characteristics than other electrolytic capacitors wherein the cathode layer is prepared from an electrolyte or MnO.sub.2. These capacitors are presently used in various electronic devices.
A description will be given of an example of conventional process for producing solid electrolyte capacitors.
First, an anode lead is attached to an anode body, which is then subjected to an electrolytic oxidation treatment to form a dielectric oxide film over the anode body. A cathode layer of solid conductive substance is formed over the oxide film. Over the cathode layer are formed a carbon layer and subsequently a silver paste layer, whereby a capacitor element is completed. A metal terminal plate is attached to each of the anode lead and the silver paste layer of the capacitor element. The assembly is then covered with a resin as by injection molding over the capacitor element and partly over the metal terminal plates to form a shell and completely fabricate a solid electrolyte capacitor.
Generally with electrolytic capacitors, damage caused to the dielectric oxide film during the fabrication process leads to increased leakage current. To reduce the increased leakage current, the capacitor element as encased in the shell is treated for aging. For the aging treatment, a dc voltage is applied to the capacitor element, which is encased in the shell, in an environment of the highest temperature at which the capacitor is to be used (or higher temperature), whereby the dielectric oxide film is restored to diminish the capacitor leakage current.
However, in the case where MnO.sub.2, conductive organic compound or like solid conductive substance is used for the cathode layer, the aging treatment encounters difficulty in restoring the dielectric oxide film and is less effective to reduce the leakage current in this case than when the electrolyte is used. Consequently, the ratio of rejects with a leakage current value exceeding the required level can not be limited to a low value.
The present inventor has carried out various experiments on organic solid electrolyte capacitors and consequently found that the leakage current can be remarkably diminished by conducting the aging treatment not after but before forming the shell. This finding has matured to the following solution to the foregoing problem.
An object of the present invention is to provide a process for producing organic solid electrolyte capacitors which are reduced in leakage current, and an apparatus suitable for practicing this process.